Smartmin

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* Stop embedding refresh code on pages that don't have View.refresh set

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* Add support for Django 2.0, drop support for Django 1.9 and Django 1.10
* Drop support for Python2

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* Create SmartminTestMixin https://github.com/nyaruka/smartmin/pull/116
* add 'create_anonymous_user' method that will ensure that anonymous user exists

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* Fix Python 3 exception usage https://github.com/nyaruka/smartmin/pull/114

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* Fix Python 3 issue with comparing ints and None https://github.com/nyaruka/smartmin/pull/113

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* Fix collect_sql when objects of different types have same name https://github.com/nyaruka/smartmin/pull/111